a closer look at componentry upper limb prosthetics

7/27/2019 A Closer Look at Componentry Upper Limb Prosthetics

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A closer look at ComponentryUpper limb prosthetics

When it comes to upper limb amputation and subsequent prosthetic fitting, little is understoodabout this very specialised procedure. The purpose of this article is to explain the types ofprosthesis available, and to outline major advances in the last ten years.

The arm is second only to the face in the amount of neural activity required to carry outactivities associated to daily living. In the past, prosthetic replacement has only been able toslightly compensate for the loss of such a complex organ, but as the age ofmechatronics(combination of mechanics and electronics) advances we are seeingprostheses with increased abilities become available to upper limb amputees.(1)

CAUSES OF UPPER LIMB LOSSThe two most common causes of upper limb loss are:Trauma mostly secondary to road or work related injuries. Trauma usually equates to 75percent of upper limb lossSurgery - performed to remove a diseased or non functional part of the body.

These amputations occur at varying levels, with these levels being summarized below;

Fore-quarter amputation, occurring higher than shoulder

Shoulder disarticulation, occurring at shoulder joint Trans-humeral amputation, occurring between shou lder and elbow joints

Elbow disarticulation, occurring at elbow joint

Trans-radial amputation, occurring between elbow and wrist

Wrist disarticulation, occurring at wrist joint Trans-carpal amputation, occurring between wrist and knuck les

Partial hand, an incomplete loss of hand

Finger amputations

TYPES OF PROSTHESES

1. NO PROSTHESISExperience has shown that those upper limb amputees who do not wear a prosthesis haveexperienced either of the following;

Bad First Experience (poor comfort or poorly made)

Unnatural Look

Reactions from Others

Development of One-Handedness

Financial Concerns Unaware of Options

Limited Functional Ability

Lack of Sufficient Prosthetic Training

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2. PASSIVE FUNCTIONAL OR COSMETIC

A passive functional or cosmetic arm is a lightweight, low function prosthesis that is used forthe following situations:- Cosmetic only arm- Lightweight- Inexpensive (NON-custom silicone)- Provides opposition to sound arm

3. BODY POWERED / CONVENTIONAL

A body powered or conventional prosthesis is an arm that utilizes a harness and cabling tocontrol a hand (also known as a terminal device) and an elbow unit. The harness system iscontrolled by gross body movements (scapular abduction, chest expansion, shoulder

depression, extension, abduction, flexion, etc).

Advantages of this type of system are;

- Heavy Duty Construction- Proprioception- Less Expensive- Lighter in Weight- Reduced Cost and Maintenance

Whilst the disadvantages are;

- Grip Force of terminal device is limited to shoulder strength and rubber band/spring

tolerance.- Range of motion is reduced due to harnessing.- Poor Cosmesis due to harnessing and cable

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- Possible Over-Use, Nerve Entrapment Syndrome due to harness

4. MYOELECTRIC / EXTERNAL POWER

All muscles are powered by a very small electric charge that runs down the nervous systemfrom the brain to the muscle to be activated. A myoelectric prosthesis utilizes this electriccharge to control either a terminal device, a wrist or an elbow unit. Small electrodes areplaced over certain muscle bellies, and their job is to pickup these charges and translate theminto movement of the specific prosthetic component.

Advantages of this type of system are- Greater Range of movement due to no external harnessing required- Increased Cosmesis- Greater Grip Force

Disadvantages of this system are- Increased Cost and Maintenance (initially)- Increased Weight (typically)- Battery charging and reduced water resistance

WHERE TO FROM HERE

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In the last ten years we have seen two important factors play out when it comes to intelligentprosthesis;1. reduced size with increased power in battery technology, and2. reduced size with increased performance of central processing units.

What this means is that prosthetic manufacturers are able to have smaller, lighter and quicker

components available to upper limb amputees. In the last number of years we have seendevices like the I-Limb from Touch Bionics and the Dynamic Arm from Otto Bock beingreleased that are superior in function when compared to older componentry. For moreinformation regarding Intelligent prosthesis please look at the websites listed below.

www.touchbionics.comwww.ottobock.com

1 Leonard, J .A., Meier, R.H. (1988). Prosthetics. In J .A. DeLisa (Ed.), RehabilitationMedicine Principles and Practice, Philadelphia, PA: J B Lippincott.

Kevin Harrison

Published April 2008

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a closer look at componentry upper limb prosthetics

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